Color image sensor and method of manufacturing the same

ABSTRACT

A color image sensor including an array of pixels is formed in a semiconductor layer having a back side that receives an illumination. Insulated conductive walls penetrate into the semiconductor layer from the back side and separate the pixels from one another. For each pixel, a color pixel penetrates into from 5 to 30% of a thickness of the semiconductor layer from the back side and occupies at least 90% of the surface area delimited by the walls. An electrically-conductive layer extends from the lateral wall of the filter all the way to the walls.

FIELD OF THE INVENTION

The present disclosure relates to a color image sensor and to a methodof manufacturing the same.

BACKGROUND

Color image sensors comprising a network or an array of pixels formed ina semiconductor layer having a first side, called a front side, coatedwith an interconnection structure and having a second side, called aback side, intended to receive an illumination, are known. In such colorimage sensors, each pixel is separated from the adjacent pixels byinsulating structures extending vertically between the front side andthe back side of the semiconductor layer. Each pixel is topped with acolor filter arranged above the back side of the semiconductor layer.The color filters of adjacent pixels are positioned side by side. Toconcentrate in each pixel the received light intensity, each colorfilter is currently topped with a microlens.

A disadvantage of such color image sensors is that a light ray having atleast partially crossed the color filter corresponding to a given pixelmay sometimes reach a neighboring pixel. Another disadvantage of suchsensors is that the materials forming two adjacent filters of differentcolors may mix.

SUMMARY

The present disclosure is directed to a method of manufacturing a colorimage sensor overcoming at least some of the disadvantages of existingsensors.

Thus, one embodiment provides a method of manufacturing a color imagesensor comprising an array of pixels formed in a semiconductor layer,with the method comprising the successive steps of etching insulatingtrenches penetrating into the semiconductor layer and separating thepixels from one another; depositing an insulating coating and anelectrically-conductive material filling the insulating trenches; foreach pixel, etching a cavity crossing the electrically conductivematerial, penetrating into from 5 to 30% of the thickness of thesemiconductor layer, and occupying at least 90% of the surface areadelimited by the insulating trenches; and filling each cavity with acolor filtering material.

The electrically-conductive material may be opaque to light. Theelectrically-conductive material may be a metal. Anelectrically-conductive coating layer opaque to light may be depositedon the insulating coating before the deposition of theelectrically-conductive material, and the cavity may be etched throughthe conductive coating layer. The conductive coating layer may be madeof a metal or of a metal nitride. The insulating trenches may penetrateinto three quarters at least of the thickness of the semiconductorlayer.

Another embodiment provides a color image sensor comprising an array ofpixels formed in a semiconductor layer having a back side intended toreceive an illumination; insulated conductive walls penetrating into thesemiconductor layer from its back side and separating the pixels fromone another; and for each pixel, a color filter penetrating into from 5to 30% of the thickness of the semiconductor layer from its back sideand occupying at least 90% of the surface area delimited by the walls,and an electrically-conductive layer extending from the lateral wall ofthe filter all the way to the walls.

The walls may comprise an electrically-conductive material coated withan insulating layer, and the electrically-conductive layer may be madeof the electrically-conductive material. The electrically-conductivematerial may be opaque to light. The walls may penetrate into threequarters at least of the thickness of the semiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages will be discussed indetail in the following non-limiting description of specific embodimentsin connection with FIGS. 1 to 7, which are simplified cross-sectionalviews illustrating steps for manufacturing an example color imagesensor.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numeralsin the different drawings and, further, the various drawings are not toscale. In the following description, terms qualifying position such asterms left-hand, top, higher, lower, vertical, lateral, etc., refer toconcerned elements in the corresponding drawings.

FIG. 1 is a simplified cross-sectional view showing a structure at anintermediate step of a method of manufacturing a color image sensor. Thestructure comprises a P-type doped semiconductor layer 1. The frontsurface (the lower surface in FIG. 1) of semiconductor layer 1 is coatedwith an interconnection structure 3 formed of portions of conductivelayers connected by conductive vias and separated by insulating layers.Interconnection structure 3 is laid on a support, or handle, which isnot shown. The back side of semiconductor layer 1 (the upper surface inFIG. 1) is coated with a stack 5 of insulating layers, for example, asilicon oxide layer 5A and a silicon nitride layer 5B laid on layer 5A.Semiconductor layer 1 comprises an array of pixels and components, notshown, such as transistors formed on its front side. Two adjacent pixels7A and 7B each comprising a photodiode 9 are shown. Each photodiode 9comprises a vertical stack of doped layers, for example, successivelycomprising, from the front side of the semiconductor layer 1, aheavily-doped P-type layer 11 (P⁺), an N-type doped layer 13, and aportion of P-type doped semiconductor layer 1. Shallow insulating areas15, for example, made of silicon oxide, are shown on the front side ofthe semiconductor layer 1 at the limit of each of the pixels 7A and 7B.

As an example, semiconductor layer 1 is made of silicon. Thesemiconductor layer may be a thin layer having a thickness from 3 to 6μm, for example, 4 μm. Silicon oxide layer 5A is, for example, a thermaloxide layer having a thickness from 5 to 10 nm, for example, 7 nm. Thethickness of the silicon nitride layer 5B may be from 50 to 60 nm, forexample, 55 nm. In a top view, each pixel 7 may have the shape of asquare having a side length from 0.9 to 2 μm, for example, 1.5 μm.

FIG. 2 is a simplified cross-sectional view showing the structure ofFIG. 1 after the deposition and the etching of a masking layer 17 on theinsulating stack 5 and after the etching of deep insulation trenches 19,or DTI (Deep Trench Isolation), crossing insulating stack 5 andpenetrating into the semiconductor layer 1. The deep insulating trenches19 penetrate into the semiconductor layer 1 across at least half andpreferably three quarters of its thickness, or also across its entirethickness. The trenches 19 separate the adjacent pixels 7A and 7B andare aligned with shallow insulation areas 15.

As an example, the trenches 19 may have a width from 150 to 250 nm, forexample, 200 nm. The masking layer 17 may be a silicon oxide layerhaving a thickness from 100 to 300 nm, for example, 200 nm.

FIG. 3 is a simplified cross-sectional view showing the structure ofFIG. 2 after successive depositions across its entire surface exposed onthe back side of the semiconductor layer 1. Such successive depositionscomprise the deposition of an insulating layer 21 and of anelectrically-conductive layer 23 on the walls and the bottom of trenches19, followed by the deposition of an electrically-conductive layer 25filling the trenches 19 to form insulated conductive walls 27. Thethickness and the material of layer 23 and/or of layer 25 are selectedso that at least one of the conductive layers 23 and 25 is opaque to thelight which will be received by the sensor in operation.

As an example, insulating layer 21 is formed by conformal deposition ofsilicon oxide with a thickness from 7 to 15 nm, for example, 10 nm. Thematerial of the electrically-conductive layer 23 may be a metal, forexample, titanium or tantalum or a metal nitride, for example, titaniumnitride or tantalum nitride. The thickness of the conductive layer 23 isfrom 10 to 50 nm, for example, 30 nm. The material of the conductivefilling layer 25 is preferably a metal, for example, tungsten.

FIG. 4 is a simplified cross-sectional view showing the structure ofFIG. 3 after the partial removal by etching of the layers 25, 23, 21,and 17 arranged above insulating stack 5, with the silicon nitride layer5B being used as an etch stop layer. Above each pixel 7A, 7B, layers 25,23, 21, and 17 are removed from at least 90% of the surface areadelimited by the walls 27. Portions 29 of the stack of layers 17, 21,23, and 25 are left in place above the walls 27 and laterally protrudeon either side of the walls 27. A portion 31 of the stack of layers 17,21, 23, and 25 is also left in place above a portion of thesemiconductor layer deprived of pixels and of the walls 27 (on theleft-hand side of FIG. 4). Although this has not been shown in FIG. 4,there is a continuity zone between the conductive layers 23 and 25 ofthe stack portions 29 and 31.

At the step illustrated in FIG. 5, etched above each pixel 7A, 7B is acavity 33 having edges aligned with those of the portions 29 of thestack of layers 17, 21, 23, and 25. Each cavity 33 crosses conductivelayers 23 and 25, and penetrates into from 5 to 30% of the thickness ofthe semiconductor layer 1. A silicon nitride layer 35 enriched withsilicon has then been deposited over the entire exposed surface area ofthe structure, on the back side of the semiconductor layer. As anexample, the thickness of silicon nitride layer 35 is from 10 to 60 nm,for example, 15 nm.

At the step shown in FIG. 6, a metal contact 37 with the conductivelayer 23 has been formed at the level of portion 31 of the stackedlayers 17, 21, 23, and 25 by etching a trench crossing layers 35 and 25all the way to the conductive layer 23 and then by depositing a metal,for example, aluminum, filling this trench. The contact 37 may be formedat the same time as insulated connections running from theinterconnection structure 3 up to the back side of semiconductor layer1. Such insulated connections are formed by etching openings from theback side of the semiconductor layer 1 all the way to theinterconnection structure 3, by depositing an insulating layer, forexample, silicon oxide, on the entire exposed surface of the structureon the back side of the semiconductor layer, and by depositing a metalin these openings. In this case, the metal of contact 37 and of theinsulated connections may be simultaneously deposited.

At the step illustrated in FIG. 7, cavity 33 opposite pixel 7A has beenfilled with a color filtering material 39A corresponding to a firstcolor, and cavity 33 opposite pixel 7B has been filled with a colorfiltering material 39B corresponding to a second color. The materials39A and 39B are flush with the level of the surface of layer 35 coveringportions 29 of the stack of layers 17, 21, 23, and 25. As an example,materials 39A and 39B are polymer resins (colored or not). At a step,not illustrated, a microlens may be arranged on each filter 39A and 39B.

The structure of FIG. 7 forms a color image sensor where each pixelformed in a semiconductor layer is topped with a color filter partiallyburied in this semiconductor layer, each pixel and its filter areseparated from the neighboring pixels and filters by insulatedconductive walls.

A first advantage of such a sensor is that two neighboring filters areseparate, which enables to avoid, on forming of the color filters, apossible mixing of the materials forming the filters.

A second advantage is that the filters are formed in cavities, wherebythe filters bond better to the sensor than in the case where the filtersare formed on a planar surface. This better bonding of filtersfacilitates the resolution of colored resins, particularly for smallpixels smaller than 1 μm.

In operation, the color image sensor is illuminated on the back side ofthe semiconductor layer 1 and the layer 35 is used as an antireflectionlayer. Further, the conductive layers 23 and 25 of walls 27 may bebiased via contact 37. Due to the fact that, as described in relationwith FIG. 3, conductive layers 23 and 25 are opaque to the lightreceived by the sensor, when a light ray obliquely reaches the uppersurface of the color filter of a given pixel, this ray remains confinedbetween the walls 27 delimiting this pixel. Further, when a light rayreaches the upper surface of a portion of stack 29 between twoneighboring filters, the ray does not penetrate into semiconductor layer1.

Thus, a third advantage of such a sensor is that only light rays havingcrossed the color filter of a given pixel reach the photodiode of thispixel. This results in an improvement of the images acquired by such asensor.

A fourth advantage of this sensor is that, due to the fact that themicrolenses topping the color filters are essentially used to avoid alight ray to successively cross two different adjacent color filters,such microlenses may be suppressed, which decreases the complexity andthe manufacturing cost of the sensor.

Specific embodiments have been described. Various alterations andmodifications will occur to those skilled in the art. In particular, thepresent invention is not limited to pixels comprising the photodiodes 9such as described in relation with FIG. 1. For example, the photodiodesmay comprise no P⁺ layer 11, or also, the photodiodes may extendlaterally all the way to the walls 27.

The number of layers and the materials of the layers of the insulatingstack 5 may be different from what has been described in relation withFIG. 1. It is also possible not to provide an insulating stack 5 on theback side of semiconductor layer 1.

The insulating structures separating adjacent pixels of the sensor maybe adapted by those skilled in the art. For example, the shallowinsulation areas 15 formed on the front side of the semiconductor layermay be omitted. A doping step may also be provided before the filling ofthe trenches 19 to form a heavily-doped layer bordering walls 27 in thesemiconductor layer 1. The insulating layer 21 may be formed bysuccessive depositions of a plurality of insulating layers, for example,a silicon oxide layer and a silicon nitride layer.

Although, in the previously-described color image sensor, only twoadjacent pixels 7A and 7B associated with two different colors have beenshown, in practice, a color image sensor comprises a very large numberof pixels, for example, several million, and more than two filter colorsare currently provided, for example, three.

Such alterations, modifications, and improvements are intended to bepart of this disclosure, and are intended to be within the spirit andthe scope of the present invention. Accordingly, the foregoingdescription is by way of example only and is not intended to belimiting. The present invention is limited only as defined in thefollowing claims and the equivalents thereto.

The invention claimed is:
 1. A color image sensor comprising: asemiconductor layer having a back side to receive an illumination; anarray of pixels in said semiconductor layer; insulated conductive wallspenetrating into said semiconductor layer from the back side andseparating the pixels from one another; and for each pixel, a colorfilter penetrating into from 5 to 30% of a thickness of saidsemiconductor layer from the back side and occupying at least 90% of asurface area delimited by the insulated conductive walls, and with anelectrically-conductive layer extending from a lateral wall of the colorfilter to the insulated conductive walls.
 2. The color image sensor ofclaim 1, wherein the insulated conductive walls comprise anelectrically-conductive material coated with an insulating layer, andthe electrically-conductive layer comprises the electrically-conductivematerial.
 3. The color image sensor of claim 2, wherein theelectrically-conductive material is opaque to light.
 4. The color imagesensor of claim 2, wherein the insulated conductive walls penetrate intoat least three quarters of a thickness of said semiconductor layer. 5.An image sensor comprising: a semiconductor layer having a back side toreceive an illumination; an array of pixels in said semiconductor layer;insulated conductive walls penetrating into said semiconductor layerfrom the back side and separating the pixels from one another; and foreach pixel, a filter penetrating into from 5 to 30% of a thickness ofsaid semiconductor layer from the back side and occupying at least 90%of a surface area delimited by the insulated conductive walls, and withan electrically-conductive layer extending from a lateral wall of thefilter all the way to the insulated conductive walls.
 6. The imagesensor of claim 5, wherein the insulated conductive walls comprise anelectrically-conductive material coated with an insulating layer, andthe electrically-conductive layer comprises the electrically-conductivematerial.
 7. The image sensor of claim 6, wherein theelectrically-conductive material is opaque to light.
 8. The image sensorof claim 5, wherein the insulated conductive walls penetrate into atleast three quarters of a thickness of said semiconductor layer.